1. Field of the Invention
The present invention relates generally to a communication method and apparatus for a downlink in a mobile communication system, and in particular, to a communication method and apparatus for transmitting and receiving common control channels in a mobile communication system.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) technology is widely being applied as a technology for broadcasting and mobile communication systems. OFDM technology cancels interference between multi-path signal components existing in mobile communication channels, and ensures orthogonality between multi-access users. In addition, OFDM technology enables efficient use of frequency resources, making it a technology suitable for high-speed data transmission and broadband systems.
FIG. 1 illustrates a structure of an OFDM signal in time and frequency domains. Referring to FIG. 1, one OFDM symbol 100 includes N subcarriers 102 in terms of the frequency domain. Individual modulation symbols 104 of transmission information are simultaneously carried on each of the subcarriers 102 in parallel. As stated above, OFDM technology, a multi-carrier transmission technology, can carry transmission data and control channel information on several subcarriers on a distributed basis for parallel transmission.
In FIG. 1, reference numerals 106 and 108 indicate start points of ith and (i+1)th OFDM symbols, respectively. In an OFDM-based mobile communication system, each physical channel is composed of one or more subcarrier symbols 104. One subcarrier interval within one OFDM symbol interval is called a “Resource Element (RE) 106” herein.
In the mobile communication system, for demodulation of received data and control information, synchronization and a cell search should first be established between a transmitter and a receiver. The downlink synchronization and cell search process refers to a process of determining a frame start point of physical channels transmitted from a cell to which a User Equipment (UE) belongs, and determining a cell-specific scrambling code applied during transmission of the physical channels. This process is referred to herein as a “cell search process”, for short. The cell search process performed by detecting a downlink Synchronization Channel (SCH) code by a UE. The UE acquires synchronization between a transmitter and a receiver and a cell Identifier (ID) for demodulation of data and control information through the cell search process.
The UE, after a success cell search, decodes a Broadcasting Channel (BCH), which is a common control channel for transmission of system information. The UE obtains system information for the cell through reception of the BCH. The system information includes information necessary for transmitting and/or receiving data channels and other control channels, such as cell ID, system bandwidth, channel setup information, etc.
FIG. 2 illustrates an OFDM-based downlink frame structure of Enhanced Universal Terrestrial Radio Access (EUTRA), which is a standard for the next generation mobile communication technology of the 3rd Generation Partnership Project (3GPP), and transmission points of synchronization channels.
As illustrated in FIG. 2, a 10-ms radio frame 200 includes 10 subframes 206, each of which includes 2 slots. Generally, 7 OFDM symbols 205 are formed in one slot 201. In the downlink, SCH is classified into two types: Primary Synchronization Channel (P-SCH) 203 and Secondary Synchronization Channel (S-SCH) 204. The SCHs are transmitted in the last two OFDM symbol intervals within the slots 201 and 202.
Similarly, the BCH carrying system information is also classified into a Primary BCH (P-BCH) and a Dynamic BCH (D-BCH). The P-BCH, a channel that a UE first receives from the SCHs after initial cell search, transmits the core system information that the UE must receive before the D-BCH.
However, most system information transmitted on the P-BCH is generally the type of information that rarely changes over time, and a Transmission Time Interval (TTI) of the P-BCH can be greater than a timing of a frame 200, the synchronization of which the UE acquires by means of the SCH. The term “TTI” as used herein is a period in which a channel coding block generated by channel-coding information transmitted on the P-BCH is transmitted. For example, although 10-ms frame synchronization is acquired through the SCH-based cell search, the channel coding block of P-BCH may be 40-ms TTI long as it is transmitted over 4 frames. In this case, the UE should acquire even a timing of the 40-ms TTI in order to normally decode the P-BCH. Therefore, there is a demand for a common control channel transmission and/or reception method and apparatus capable of acquiring a timing of the P-BCH and decoding P-BCH information with low complexity, even when a TTI of common control channels is greater than an interval of frame synchronization acquired through synchronization channels in the mobile communication system.